An article entitle "Leaking-mode propagation in Ti-diffused LiNbO.sub.3 and LiTaO.sub.3 waveguides," published in Optics Letters, Vol. 3, No. 3, September 1978, relates the observation of leaking-mode propagation in optical waveguides. In particular, it was reported that anisotropic coupling which occurs between TE and TM polarizations in nonaxial propagation directions causes one of the propagating modes selectively to be leaking, with consequent high propagation losses for such mode. In particular, it was reported that in an X-cut or Y-cut lithium niobate waveguide high propagation loss occurs selectively for only a TE-polarized input. In X-cut or Y-cut lithium tantalate waveguides, the TM-polarized mode is selectively affected instead. In particular, for the effect to be significant, the propagation direction needs to be offset from parallelism with the optic or Z-axis of the crystal by an angle larger than a critical angle for the particular crystal. The critical angle is a function of the waveguide and the particular crystal. It typically may be several degrees and is best determined experimentially for a particular waveguide design. It can be modulated by electric fields set up in the waveguide.
For example, for waveguide angles greater than the critical angle, when a TE-polarized 6328 Angstrom beam was applied as an input to a lithium niobate waveguide for propagating angles along the waveguide greater than the critical angle, there was observed not only the guided beam with TE-polarization, but also a leaking beam, originating along the waveguide but propagating into the substrate with a tilt angle relative to the waveguide surface, and the leaking beam was found to be TM-polarized, rotated 90.degree. from the input beam. No leaking and so no loss was observed for a TM polarized input. A modulator that was designed to harness this effect included a planar optical waveguide placed at an angle to the optic axis of the crystal as close to the crictical angle as feasible. The critical angle was then modulated by electro-optic perturbation to vary the loss of the input TE mode. Because of the necessity of closely controlling the direction of wave propagation to keep it close to the critical angle, the modulator employed a prism coupler to launch accurately the input wave into the waveguide. This technique is accordingly complex and not especially well suited for use with transmission systems in which an optical fiber is used as the principal optical transmission medium.
The present invention is directed at a modulator which is better adapted for use with optical fibers for coupling to the modulator.